(a+b)^2-(2a-b)^2
=[(a+b)-(2a-b)][(a+b)+(2a-b)]
=(a+b-2a+b)(a+b+2a-b)
=3a(2b-a)
x^3-x
=x(x^2-1）
=x(x-1)(x+1)
2x^4-18
=2(x^4-9)
=2(x^2-3)(x^2+3)